This invention relates to polymer substrates for radiation-induced graft polymerization that are made from polymer fibers, particularly from polyethylene fiber, as well as radiation graft treated stock that is produced by introducing functional radicals into the substrates by means of radiation-induced graft polymerization. The invention also relates to an improvement in the method of radiation-induced graft polymerization that is applied to polymer substrates in the form of a woven or non-woven fabric.
Radiation-induced graft polymerization is a technique by which a substrate polymer is irradiated to form radicals and a polymerizable monomer is grafted onto the radicals. Since functional radicals can be introduced into various shapes of high-molecular weight compounds, radiation-induced graft polymerization is drawing increasing attention these days as a process for producing materials having a separating capability. The attention is especially significant when it comes to methods of producing stock for air purifying chemical filters which have recently seen increasing use in purifying the air in clean rooms employed in precision electronics such as semiconductor fabrication and in the manufacture of pharmaceuticals, as well as to methods of producing stock for ion-exchange filters used in the production of pure water.
Polyolefinic high-molecular weight materials are considered to be suitable as polymeric substrates for radiation-induced graft polymerization. Among these, polyethylene is considered the best stock for radiation-induced graft polymerization. This is because polyethylene is easier to crosslink but more resistant to decay after exposure to radiation as compared to other polyolefinic materials. Polyethylene-based substrates for radiation-induced graft polymerization are well known in the form of film and hollow yarn, which are used as ion-exchange membranes, cell diaphragms, air purifying materials, affinity separating membranes, water treating materials, deodorants, etc. A case of producing a cell diaphragm using a film treated by radiation-induced graft polymerization is disclosed in YUASA JIHO, 54, 57-62 (1983) under the title xe2x80x9cOn Membranes Produced by Pre-irradiation Graft Polymerizationxe2x80x9d. The use of shaped polymers as water treating materials is disclosed in Japanese Patent Public Disclosure Nos. 111685/1993 and 111637/1993.
Forming woven or non-woven fabrics from the fibers of polymers such as polyolefins and polyesters and using them as filter stock is a common practice; however, as far as the present inventors know, commercial use of polyethylene monofilament fiber as the raw material for filter stock in the form of a woven or non-woven fabric as a fiber aggregate has been very scarce. This is because the physical and chemical characteristics, such as the melting point and chemical resistance, of polyethylene are inferior to those of other polyolefinic materials typified by polypropylene, so the use of polyethylene monofilament fiber has not drawn much attention as a candidate for filter stock. In fact, the polyethylene monofilament fiber has outstanding characteristics for radiation-induced graft polymerization and it can be processed into woven or non-woven fabrics, into which functional radicals are introduced by radiation-induced polymerization. However, the woven or non-woven fabric materials thus produced by radiation-induced graft polymerization do not have high enough physical strength, so they undergo permanent set strain, commonly called xe2x80x9cfailurexe2x80x9d, and are unable to maintain sufficient strength and dimensional stability to function as filter stock. As the result, considerable difficulty has been encountered in molding them into a pleated filter or the molded filter experiences increased pressure loss.
With a view to solving these problems, the present inventors proposed the production of fiber having improved separating capability by applying radiation-induced graft polymerization to a core/sheath composite fiber (Japanese Patent Public Disclosure No. 199480/1996). To make the proposed fiber having improved separating capability, a composite fiber using high-melting point polymers such as polyethylene (in the sheath) and polypropylene or polyethylene terephthalate (in the core) is employed as the substrate for grafting and this enables a thermal fusion method to be practiced at the stage of processing into a non-woven fabric. As a result, the physical strength of the core is combined with the force of adhesion created at the points of contact between individual filaments and the fiber exhibits a very significant physical strength.
A problem with this composite fiber is that graft polymerization occurs primarily in the sheath-forming polyethylene, so the sheath occasionally separates from the core after graft polymerization to create gaps, in which processing chemicals stay to induce deterioration of the fiber characteristics for the cleaning step in the manufacturing process. If an attempt is made to increase the graft ratio of the composite fiber taken as a whole, the graft ratio of the sheath increases so much that its breakdown occurs, though on rare occasions.
The present inventors conducted intensive studies with a view to enhancing the strength of the stock that was to be formed by applying a radiation-induced graft polymerization treatment to a substrate in the form of a woven or non-woven fabric composed of polyethylene monofilament fiber. As a result, it was found that when a substrate in the form of a woven or non-woven fabric composed of polyethylene monofilament fiber was combined with a reinforcement polymer having a greater strength and a slower rate of graft polymerization than the polyethylene monofilament fiber, the physical strength of the polyethylene woven or non-woven fabric material was enhanced; it was also found that by applying radiation-induced polymerization to said material, filter stock having improved capabilities and strength characteristics was produced. It was further found that this filter stock had outstanding advantages that were quite unexpected as will be set forth hereinafter.
Thus, according to its first aspect, the present invention relates to a polymer substrate for radiation-induced graft polymerization in the form of a woven or non-woven fabric that comprises a woven or non-woven fabric composed of polyethylene fiber and a reinforcement polymer having a greater strength and a slower rate of graft polymerization than said polyethylene fiber.
The invention also relates to filter stock that has functional radicals introduced into the substrate for radiation-induced graft polymerization according to the first aspect by radiation-induced graft polymerization.